Human African trypanosomiasis

Vector control

Current vector control interventions involve the use of insecticides either through sequential aerosol spraying technique (SAT); ground spraying; insecticide-treated targets or insecticide-treated animals – live baits; the use other-baited traps or screens, and the sterile insect technique (SIT).

Some of the interventions conducted in the past such as bush clearing (tsetse habitat destruction) or elimination of wild animals (tsetse reservoir hosts) have been discarded for ecological and environmental concerns. Extensive insecticide ground spraying which was used to control tsetse in Zimbabwe and Nigeria is seldom used today because of concerns over residual insecticides and because the operational demands are beyond the means of most African governments. SAT, which can effectively clear large areas of tsetse in a relatively short time, requires substantial economic and infrastructure support. Recent advances in aircraft guidance systems have considerably increased the accuracy and the efficiency of insecticide delivery as shown in recent control operations in the Okavango delta in Botswana and Namibia. SAT has recently been undertaken in Ghana and Burkina Faso under the respective country PATTEC projects. Aerial spraying is also ongoing in Angola and Zambia. The main limitation of aerial spraying as with other methods is the re-invasion pressure if the area is not isolated. Pour-ons (application of insecticides to backs of cattle on which tsetse feed) or selective spraying on legs and belly where flies go selectively to bite are another effective means of vector control saving funds and minimizing the distribution of pesticide in the environment.

Odor baited traps and screen impregnated with insecticide and appropriate attractive colours have been used in many countries to effectively suppress tsetse population by 99%. These artificial bait methods are cheaper than ground and aerial spraying but communities and governments cannot deploy them on sustainable bases, as they are labor and management intensive. While, the simplicity of the traps or targets lends them to be used by local communities, sometimes these are applied on a scale so small that control efforts are bound to be frustrated by re-invasion. While effective baits have been developed for savannah tsetse no such baits exist for riverine tsetse major vectors of HAT. Recent research is also showing that for riverine tsetse targets of much smaller size could be effectively used as compared to those for savahhah flies. Studies to develop odour baits for riverine tsetse fromrepitilian hosts are in progress. Synthetic repellents and repellents from animals not preferred by tsetse (such as waterbuck) have also been identified and are currently ongoing large scale validation trials.

The Sterile Insect Technique (SIT) is another approach to reduce tsetse populations. This technique is based on the fact that females mate only once in their lifetime, thus any mating with a sterile male will prevent females from giving birth to any offspring. SIT consists in rearing a large numbers of laboratory male tsetse flies which are irradiated and subsequently released in the wild to compete with wild (naturally occurring) males so that females inseminated by them produce no off-spring. The technique has been effectively used for eradication of tsetse (G. Austeni) from Unguja island in Zanzibar. Effective suppression using conventional methods is a pre-requisite before this environmentally safe species-specific technique can be used to eradicate residual populations. Ecological islands are needed for the success of this technique or else re-invasion is bound to occur. The cost of SIT is, however, exorbitant and the mass rearing of the flies a major problem as large of numbers of male flies are needed to be released to out-compete the wild population. This technique is also logistically and management intensive and its feasibility in areas where multiple species are present remains doubtful.

Recent advances in molecular technologies and the availability of genomic information could lead to the development of new control strategies aimed directly at the fly or its parasite transmission ability. Genomics of tsetse symbiotic bacteria are of interest since in the absence of their gut flora; tsetse flies are severely impaired in their longevity and reproduction. Two bacteria have been implicated in modifying vector competence of their host (Sodalis golssinidius and Wigglesworthia glossinidia). A third symbiont, Wolbachia can confer mating sterility. Such transgenic refractory flies could be released into natural populations to replace their susceptible counterparts and hence reduce disease transmission. They could also be immediately used in SIT release programmes and reduce the cost of the projects and the increase the efficacy of their application in HAT endemic areas.

Knowledge of olfactory genes may result in development of more potent attractants and repellents. The use of several methods in an integrated disease and vector management package is generally recommended. An integrated pest management approach is preferable because it exploits all the weaknesses in fly behavior and enables synergies of the methods that increase the intensity of the control effort. Since reinvasion of cleared areas from adjacent un-controlled areas is a major problem due to the mobility of the flies and absence of natural barriers, an area–wide pest management approach should be undertaken.